I know this post is 5 years old, but this problem is still cropping up from time to time with no explanation. Specifically, myself and others have an issue with a large packet length of 1410, with a threshold of 1406. Unfortunately, you can't change the MTU because the maximum is 1406. This doesn't even make sense.

I did find one possibility. A troubleshooting article showing the ASA transmitting packets that exceeded the MTU could be fixed by running: